During anesthesia, and especially during sedation anesthesia or patient controlled analgesia it is often desirable to collect and analyze qualitatively and quantitatively the constituents of gases respired by the patient.
Oral/nasal cannula manifolds are used to deliver supplemental oxygen to hospital patients who require respiratory support and to collect carbon dioxide samples from the patients to monitor respiration. The oral/nasal manifold is configured to be in close proximity to the oral cavity and also inserted into the nasal cavity of the patient. The patient's exhaled breath is drawn through the manifold passages to a gas analyzer to be analyzed. The results of this analysis provides an indication of respiratory adequacy.
The accuracy of this analysis of exhaled gases depends on the ability of a sampling system to optimally move a gas sample from the patient to the gas analyzer while maintaining a smooth, laminar flow of gases, such that there are as few alterations to the waveform and response time of the concentration of the gases as possible. The waveform of the concentration of the gas is critical for accurate analysis. As the gas mixtures travel from the patient to the gas analyzer, the concentration of the gases can be effected by mixing of the component gases, which reduces the accuracy of the analysis of the sample by the gas analyzer, and reduces the amount of information obtained from the analysis.
As is pointed out in U.S. Pat. No. 6,422,240, prior art oral/nasal cannulas have caused significant alterations to these important features of the internal structure of the stream of exhaled gases. For example, alterations arise as a result of attempt to combine the delivery of oxygen with the sampling of the exhaled breath of the patient. As another example, prior art oral/nasal cannulas have cannula passages which include connected adjacent or ancillary void volumes (space which is not part of the designated pathway for the flow of gases), and in addition, the cannula passages have curved sections which provide restriction to flow of the respired gases and therefore provide different flow rates in connected cannula passages. Accordingly, the waveform of the concentration of the gas is altered and accurate analysis is not provided. There is accordingly a need to provide an oral/nasal manifold which provides optimal sampling of the subjects exhaled gases for analysis in order to provide an optimal waveform from the analyzer.
The deficiency in these prior art designs is the undesirable diminished fidelity resulting from gas sampling flow dynamics. Specifically, the amount of curvature in the cannula passages and the amount of dead or void space gases, which have limited or no carbon dioxide content, that is analyzed along with the actual ventilatory gases, results in a diluted sample which produces a lesser quality or low fidelity signal generation.
In fact, the flow path created by the drafting effect in the cannula passages is more prominent on the void or dead space gases. This facilitates the production of diluted end title carbon dioxide data and provides a lesser fidelity capnographic waveform and the curvatures and ancillary void volumes in the cannula passages prevent optimal cancellation of signal degradation caused by lack of expiration at the opposite site. In other words, the nasal and oral samples are somewhat in parallel and not in opposition with each other thereby providing degraded sampling quality.
Another deficiency in the prior art designs is that the oral collectors are inefficient and ineffective and an excessive amount of external gases are mixed with the orally exhaled gases thereby providing inaccurate readings.